1. Field of the Invention
The present invention relates to an artificial knee joint used to restore the normal function of a knee joint that has been heavily deformed due to such disease as gonarthrosis or chronic articular rheumatism.
2. Description of the Related Art
An artificial knee joint comprises a femoral component fixed onto the distal portion of the femur, and a tibial component fixed onto the proximal portion of the tibia. As an example of an artificial knee joint, such a constitution has been known as a convex sliding surface having a spherical shape or a spheroid formed on a sliding portion located at the middle on the rear end of the femoral component, and a concave surface having a spherical shape or spheroid formed on a sliding portion located at the middle on the rear end of the tibial component (for example, Japanese Unexamined Patent Publication (Kokai) No. 4-158860). The convex sliding surface and the concave sliding surface fit to each other when the knee bends or stretches, so as to make a motion of sliding rotation against each other on the rear, thereby allowing a wide range of motion. Moreover, the convex sliding surface and the concave sliding surface are enabled to fit to each other and make a motion of sliding rotation while supporting a load on the main articular surface even when bending the knee, by setting the height T1 of the convex sliding surface and the depth T2 of the concave sliding surface to satisfy a relationship of T1≦T2.
Another artificial knee joint is known which has such a constitution that comprises a tibial component and a femoral component, wherein the articular surface of the femoral component extends smoothly from the distal condyle to the epicondyle, so as to allow motion over a range of at least 160 degrees (for example, Japanese Unexamined Patent Publication (Kokai) No. 11-313845).
However, the artificial knee joint disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-158860 undergoes osseous impingement, that is contact between the remainder of the femur and the femoral component, when bending of the knee reaches 150 degrees or more (refer, particularly, to FIG. 4 of Japanese Unexamined Patent Publication (Kokai) No. 4-158860). When osseous impingement occurs, the knee cannot bend anymore. The seiza position (Japanese sitting posture on tatami) requires bending of the knee at an angle of about 160 degrees. Therefore, there have been cases in which patients having an artificial knee joint implanted who cannot sit in seiza style.
It is effective for preventing the osseous impingement from occurring to properly trim the residual bone during the ostectomy operation to resect the joint from the femur. However, such an operation of resecting the bone requires an advanced osteotomy technique, and there is a demand for an artificial knee joint that prevents the occurrence of osseous impingement more easily.
The artificial knee joint disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-313845 enables deep bending of the knee with bending angles of up to 160 degrees while compressing the occurrence of osseous impingement, by such a constitution as a spine of the tibial component and a cam of the femoral component making contact and sliding against each other. However, should osseous impingement occur when the knee bends as deep as 160 degrees, so-called soft tissue impingement is expected to occur, in which the soft tissue that surrounds the knee joint contacts the remainder of the femur 3 or the femoral component 2. It is desired to avoid the soft tissue impingement since it may damage the soft tissue. It is effective for preventing the soft tissue impingement from occurring, to design the artificial knee joint so as to be capable of bending at angles of a greater range (range of motion) than the range of bending angles which the knee actually experiences in real life movements.
Also according to Japanese Unexamined Patent Publication (Kokai) No. 11-313845, the spine of the tibial component is formed from ultra-high molecular weight polyethylene (UHMWPE), and the contact area between the spine and the cam is small during the sliding motion of the cam of the femoral component. As a result, the surface of the spine that contacts the cam is subjected to localized severe wear even in a relatively short period of use, thus making it inevitable to carry out an additional operation to replace the artificial knee joint.